Abstract
We have synthesized Cu2O-ZnO heterostructured nanocrystals by using a simple method in aqueous solution, and performed a series of photodegradation experiments. Our lab has previously demonstrated strongly facet-dependent photocatalytic properties of Cu2O cubes, octahedra and rhombic dodecahedra. We found Cu2O cubes have no activity to photodegrade methyl orange (MO) dye molecules. This result violates our general understanding of photocatalysis mechanism, so we want to extend our investigation to semiconductor–semiconductor heterojunctions by growing ZnO on Cu2O crystals. Because formation of semiconductor heterostructures with proper band alignment should improve the photogenerated electron–hole pair separation, we expect the photocatalytic activity of Cu2O cubes should remain inactive or become somewhat active with contribution from the ZnO side, while Cu2O octahedra and rhombic dodecahedra decorated with ZnO should show enhanced photocatalytic activities. Surprisingly, while Cu2O cube-ZnO composite structures remain inactive, ZnO-decorated Cu2O octahedra also become inactive. ZnO-decorated Cu2O rhombic dodecahedra show good but a slightly decreased activity possibly due to Cu2O surface coverage. The Cu2O-ZnO heterostructured nanocrystals have been characterized by various techniques, showing maintenance of their composition before and after photocatalysis, although some CuO has been formed. Through extensive analysis of interfacial HRTEM images, we found that the {101} facets of ZnO preferentially grew epitaxially on the {111} faces of Cu2O, but multiple ZnO lattice planes can grow on Cu2O cubes and rhombic dodecahedra. Assuming the (101) planes develop a large band bending preventing photoexcited electrons from migrating to the ZnO side, this unfavorable situation explains the lack of photocatalytic activity of ZnO-Cu2O octahedra. This finding has profound implication, showing that formation of unfavorable heterojunctions can lead to sharp decline in charge carrier transport ability even when band alignment of the semiconductors forming the heterojunctions predicts catalytic enhancement. This is another dramatic demonstration of semiconductor facet effects.